This invention relates to a simple, efficient and economic multiorder spectrograph based upon a modified Ebert-type mounting, using a standard low-blaze-angle plane reflection grating as the multiorder dispersing element. This invention also relates to the efficient usage of the available pixels in a modern two-dimensional detector array, such as a CCD, by filling the array area with a multiorder spectral display covering wavelengths ranging from the vacuum ultraviolet to the infrared, either in their entirety or in selected wavelength segments, at medium to high spectral resolutions.
Until recent years the photographic emulsion was typically used as the recording means in spectrographs. The advantage of the emulsion was that it provided durable data storage having an enormous number of detector elements (photographic grains) at low cost. But the low quantum efficiency (QE), and the numerous problems associated with processing and measuring photographic plates, gave rise to common usage of faster and more convenient scanning spectrometers, or monochromators, using a photomultiplier (PMT) for routine spectral measurements not requiring a large number of resolution elements.
More recently, electronic detector arrays, such as CCDs, having large numbers of pixels, and QEs significantly higher over wider wavelength ranges than even PMTs, have become the detector of choice in spectroscopy. But whereas the cost of those earlier detectors was normally a small fraction of the cost of the spectrograph, modern scientific-grade CCDs are often the most expensive part of the spectrograph system. This creates a significant need for an inexpensive high-performance spectrograph designed to make most efficient use of these powerful detector arrays, while keeping the overall cost of the spectrograph system within budget.
The vast majority of spectrographs and spectrometers continue to be used to measure spectra in one spectral order, usually the first order, with order-sorting filters used as needed to block other orders. In such applications, a square detector array having a million pixels will typically measure fewer than 500 spectral resolution elements at one time (due to the images typically overlapping 2-3 pixels). This is a serious under-utilization of the available pixels in these expensive detectors.
An obvious way to greatly improve utilization of detector arrays is to fill the pixel array with a multiorder spectrum using a grating and a cross-dispersing element. This approach is based upon the fundamental characteristic of blazed gratings, that the light diffracted in the blaze direction is comprised of radiation in a plurality of spectral orders, where the central wavelength of each order is given by xcexo /m, where xcexo is the first-order blaze wavelength and m is the spectral order. To clearly separate these orders in the image plane it is necessary to also introduce a cross-dispersing element (a prism or a second grating) to disperse this same radiation perpendicular to the first grating""s dispersion. A thorough tutorial regarding the terminology, construction and theory of diffraction gratings is given in E. Loewen , et al., xe2x80x9cDiffraction Grating Handbook, Bausch and Lomb, Inc., 1970, which is incorporated by reference as if fully set forth herein.
It is commonly assumed that multiorder spectrographs are in fact echelle spectrographs, inasmuch as essentially all multiorder spectrographs use echelle gratings. But the main purpose of echelle gratings is not that they should produce multiorder spectra, that being a necessary and often undesirable byproduct of their design. The reason and justification for using echelle gratings is the very high spectral resolution they afford as a direct result of their high blaze angles (typically 63xc2x0-76xc2x0), and the fact that angular dispersion of a reflection grating is proportional to Tan B, where B is the grating""s blaze angle. Thus, the angular dispersion of an R2 (Tan 63.4xc2x0=2) echelle is 10 times, and an R4 (Tan 76xc2x0=4) echelle is 20 times that of a typical standard plane grating having a blaze angle of 11xc2x0. But very high spectral resolution is not a common requirement in spectroscopy, as verified by the fact that the relatively costly and complex echelle spectrographs comprise only a tiny fraction of the spectrographs that are in use.
A much simpler and less expensive way to perform multiorder spectroscopy for a majority of applications, where modest resolutions over a large wavelength range is the goal, is to use a cross-dispersed low-blaze-angle grating blazed at several times the longest wavelength to be studied. Such gratings, having a wide range of blaze and dispersion characteristics, are commercially available at reasonable cost A multiorder spectrograph using such a grating was reported by R. L. Hilliard, etal., xe2x80x9cA Cross-Dispersed Echelette Spectrograph and a Study of the Spectrum of the QSO 1331+170xe2x80x9d, Ap.J., 1975, 351-361, Vol. 196.
An important requirement for any multiorder spectrograph is that the image quality over the area of the detector be comparable to or smaller than the pixel size. The spectrograph must therefore have negligible astigmatism, coma, and spherical aberration over the required field; and to avoid chromatic aberration over such a large wavelength ranges effectively requires all mirror imaging optics.
An elegant optical system uniquely meeting these requirements has its roots in the Ebert-type mounting, originally described by W. G. Fastie, xe2x80x9cA Small Plane Grating Monochromatorxe2x80x9d, J.O.S.A., 1952, 641-647, vol 42, no. 9. The popular Ebert has a single spherical mirror serving both as collimator and camera, and the plane grating is located near the sphere""s focus. Although corrected for coma, the Ebert still has astigmatism and spherical aberration, which has restricted its use to that of a scanning spectrometer. But an essentially unnoticed article by W. T. Welford, xe2x80x9cStigmatic Ebert-Type Grating Mountingxe2x80x9d, J.O.S.A., 1963, 766, vol. 53, revealed that the images would become free of aberration if the spherical mirror of the Ebert were simply replaced by a paraboloidal mirror of the same focal length. The only previous example of anyone actually using a paraboloidal mirror in an Ebert-type mounting appears to be I. Furenlid and O. Cardona, xe2x80x9cA CCD Spectrograph with Optical Fiber Feedxe2x80x9d, P.A.S.P., 1988, 1001-1007, vol. 100.
It is an object of the present invention to provide a simple, efficient, and economic multiorder spectrograph for use over a wavelength range from the vacuum ultraviolet to the infrared.
A further object of the invention is to provide such a spectrograph based upon the Ebert-type mounting, where the spherical collimator/camera mirror normally used therein is replaced by a paraboloidal mirror to eliminate the Ebert""s astigmatism and spherical aberration, and to thereby create a spectrograph that has image quality comparable to the pixel resolution over the area of a two-dimensional electronic detector array.
A further object of the invention is to provide such a spectrograph that utilizes a low-blaze-angle reflection grating having first-order blaze so that spectra at shorter wavelengths of interest shall be most efficiently diffracted into higher spectral orders.
Another object of the invention is to provide such a spectrograph wherein a cross-dispersing element is located between the reflection grating and the paraboloidal mirror where it is used to cross disperse, and thereby separate, the grating orders perpendicular to the grating dispersion, to create a multiorder spectral, display.
It is also an object of the present invention to provide such a spectrograph that can create multiorder spectral displays for simultaneous recording of very large wavelength ranges at moderate to high resolution, using two-dimensional detector arrays having Np pixels to detect as many as N less than Np/10 spectral elements in a single exposure.
Another object of the invention is to mount and permanently align the grating and cross-dispersing element together in a removable and interchangeable cross-dispersion assembly which provides the dispersion characteristics needed to project a two-dimensional spectral display of a selected wavelength region and range, at a certain spectral resolution, onto a given two-dimensional electronic detector array.
A further object is to provide such a spectrograph having a specific and dedicated collimator/camera mirror and optomechanical construction, but which by use of a plurality of said crossed-dispersion assemblies, each having its own particular grating and cross disperser, can be used to provide a variety of multiorder spectral displays of selected wavelength regions, ranges, and resolutions for use with a variety of detector arrays.
Another object of the invention is to provide the option to fill the gap between the grating and prism with a fluid of index n to thus increase the effective blaze angle of the grating by a factor of approximately n.
Furthermore, it is an object of the invention to provide a spectrograph wherein its optical functions may occur in a variety of optical media, including air, other gases, a vacuum, or any other optically transparent media of index n.
The general object of the invention is to provide a simple, efficient and economic multiorder spectrograph, particularly as compared to a typical cross-dispersed echelle spectrograph, that provides a flexible means to optimally utilize the useful area, pixel resolution and wavelength sensitivity of two-dimensional electronic detector arrays, to record spectra over large wavelength ranges at moderate to high resolutions. Other objects and advantages of the present invention will become apparent from the figures and detailed description to follow.
Although this invention uses a modified Ebert-type mounting, it will be apparent to those skilled in the art that the type of modifications made herein to achieve the objects of this invention may also be applied to similarly expand the capabilities of other optical systems such as the Czerny-Turner spectrometer.